Natural convection flow visualization experiments during melting and solidification of a hypoeutectic gallium five weight percent indium were performed using a real time X-ray radioscopy technique. The experiments revealed a conductive-convective threshold for the onset of natural convection in the alloy. The threshold appears to be caused by a substantial chemical stratification of indium in the melt. The observed gravitational segregation has been evaluated and compared to theoretical models, which, however, do not predict the observations.